Thermal printing systems use a thermal print element energized to heat specific and precise areas of a heat sensitive paper to provide an image of readable characters or graphics on the heat sensitive paper. The heat sensitive paper, also known as thermal paper, includes material(s) which are reactive to applied heat. The thermal paper is a self-contained system, referred to as direct thermal, because ink need not be applied.
Thermal printing systems are ubiquitous and typically include point of sale (POS) devices, facsimile machines, adding machines, automated teller machines (ATMs), credit card machines, gas pump machines, electronic blackboards, and the like.
Typical chemistries used in thermal papers are based on (a) silver salts combined with reducing agents that are activated by heat, and (b) dye-developing type systems that comprise a colorless dye (color former), a bisphenol such as bisphenol-a (color developer) and a sensitizer. These solid materials are reduced to very small particles by grinding and incorporated into a coating formulation along with any optional additives such as pigments, binders and lubricants. The coating formulation is then applied to the surface of a support system, typically a base sheet and base coating. The color is formed by application of heat to the thermosensitive coating to melt together and thereby cause a darkening reaction.
While the above described imaging systems produce acceptable thermographic results, they suffer from certain drawbacks. For example, silver is costly and its price is uncertain in the world marketplace. Moreover, bisphenol compounds such as bisphenol-A have been listed among the chemicals that may cause disruption of reproductive function in animals and humans. The route of entry in humans is typically through the skin and can be facilitated by oils and hand creams that may be present on the skin. Biedermann et al., Anal Bioanal Chem., 398, 571, (2010) report that a cash register worker may absorb as much as 71 micrograms of bisphenol-A in a 10-hour work day. Further, owing to the extensive use of bisphenol-A based thermographic materials, environmental impact is a concern because thermal papers are recycled along with ordinary paper. The recycled materials containing adventitious bisphenol-A may later be used as ordinary office paper or in applications such as food packaging.